The injection of unfractionated donor bone marrow cells (BMC) into skin grafted and antilymphocyte serum (ALS) treated recipient mice is a well-established protocol for the induction of specific allograft prolongation. This result is due in part to the presence of anti-donor blocking factor, a decrease in host cell mediated immunity and to donor BM derived suppressor cells. ALS and unfractionated donor BMC have been effective in prolonging renal allografts both in dogs and in one clinical trial. Utilizing the technique of velocity sedimentation at unit gravity we have isolated a particular BMC type which is active in specific skin graft prolongation in mice. This 3mm/hr sedimenting cell is morphologically a small lymphocyte, is slightly adherent to nylon wool, appears not to be a CFUs cell, is Fc-, Thy 1.2-, Ly2-, ALS+, is likely IgM+ and Ia+, is resistant to in vivo hydroxyurea treatment, is responsive to in vitro PHA and Con A stimulation and is suppressive both in MLC and in in vivo adoptive transfer experiments. Mechanistically, these results suggest that the graft prolonging BMC is a differentiated cell (or one not undergoing rapid division) of the T cell lineage which may be mediating its effect through a GVH reaction. Most importantly, 1 million of these cells are superior to 25 million unfractionated BMC in inducing skin allograft prolongation. The injection of purified and well-characterized cells eliminates the need to inject large numbers of extraneous and deleterious BMC for the induction of specific graft unresponsiveness. Proposed studies in mice involve the further identification of this cell population by affinity chromatography, rosette separation and electrophoresis, and continued elucidation of the mechanism of action of its graft prolonging effect by both in vitro culture techniques and in vivo adoptive cell transfer experiments. Since initial BMC velocity sedimentation profiles in both the dog and monkey yield a cell type morphologically similar to the one identified in the mouse, renal allografts will immediately be performed to test the in vivo graft prolonging effectiveness of this cell type. Continued dog and monkey BMC purification methods will be based on the proposed mouse experiments. Finally, human BM will be studied for the presence of an analogous graft prolonging cell type. These studies should have potential clinical benefit since they could show that relatively few isolated and well-characterized donor BMC result in extensive organ graft prolongation.